The abducens nerve (VI)

Originating between the medulla and pons, the abducens nerve passes within the cavernous sinus. It innervates the lateral rectus muscle [4–6].

Multiple disorders involve the innervation of the eyes. Optic neuritis may herald the onset of multiple sclerosis. It causes decreased visual acuity, an a erent papillary defect, and pain on eye movement. Fundoscopic examination may be normal (if the inflammation is retrobulbar) or may reveal edema of the optic disc (if the inflammation is anterior) [11].

Unilateral or bilateral abducens nerve palsy (inability to abduct one or both eyes) may be an early sign of increased intracranial pressure [12]. A posterior communicating artery aneurysm may compress the ophthalmic nerve, causing lid lag and ophthalmoplegia [12]. These entities will be discussed in detail in the article by Duong, Leo, and Mitchell found elsewehere in this issue. Furthermore, trauma can cause an orbital hemorrhage, which may lead to increased orbital pressure and ischemia of the optic nerve.

The blood supply of the eye

The first branch of the intracranial section of the internal carotid artery is the ophthalmic artery. The ophthalmic artery and its subsequent branches supply the main blood source to the eye and its structures. After the ophthalmic artery enters the orbit, the first branch is the central retinal artery. As the artery continues to course within the orbit, multiple other branches supply the structures of the eye until the most superficial branches reach the eyelids. Here they form arcades that anastomose with the external carotid artery.

The main venous drainage of the eye occurs via the superior and inferior ophthalmic veins. These two veins receive the drainage from the eye including the central retinal vein. They then empty into the cavernous sinus. The venous supply of the periorbital skin is contiguous with the deeper venous structures. Superficial skin infections can therefore spread via the venous system into the cavernous sinus and deeper structures [4–6].

The blood supply to the eye can be threatened in several ways. Embolization of plaque or clot to the central retinal artery can cause central retinal artery occlusion and acute painless monocular visual loss. Central retinal vein occlusion also causes visual loss over a longer time period than central retinal artery occlusion. Inflammation of the ophthalmic artery can result from temporal arteritis, causing painless visual loss, which may be unilateral or bilateral [11]. These disorders will be discussed in the article by Vortmann and Schneider in this issue. Also, trauma can disrupt the blood supply to the eye.

Physical examination of the eye

In performing the physical examination of the eye, one should be able to evaluate both anatomy and function of each eye. The evaluation of the anatomy should focus on whether the problem arises from the globe, the orbit,

or the external structures. The evaluation of the function of the eye should include vision, alignment, and movements. Generally, the physical examination of the eye should begin with a measurement of visual acuity (an exception to this is when ocular exposure to toxic substances requires irrigation before visual acuity measurement [13]din this case, a quick measurement of ocular pH can precede irrigation if it can be achieved in a few seconds and does not delay irrigation), then the examiner should start peripherally with the periorbital area and work centrally to examine the orbits, lids, the extraocular muscles, the conjunctiva, sclera and episclera, the uvea, the intraocular pressure, the cornea, the pupils, the anterior chamber, and the fundus.

Vision

The standard way to test vision is by reading a Snellen chart at 20 feet (Fig. 4). Each eye should be tested separately. The vision is then assigned a two-digit score (for example, 20/50) for each eye. The first number represents the distance from the chart, and the second digit represents the smallest line readable by the patient. Recording the vision separately for each eye, even if the vision is the same in each eye, is essential (for example, visual acuity: right eye 20/50, left eye 20/50). If a patient can read three letters on the next line, this can be recorded as, for example, 20/50 þ3. If a patient gets 2 letters wrong on the last line, this can be recorded as, for example, 20/50 2

Fig. 4. Standard Snellen chart. (From Kniestedt C, Stamper RL. Visual acuity and its measurement. Ophthalmol Clin North Am 2003;16:159; with permission.)

[12]. Patients who cannot read English can generally be tested with a tumbling E chart, in which the patient identifies the orientation of the letter E (facing to right, left, up, down) [14]. Testing vision in children will be discussed in detail in the article by Prentiss and Dorfman in this issue.

If the subject wears glasses and they are unavailable, vision can be tested through a pinhole. By reading the Snellen chart through multiple pinholes, the image is transmitted centrally through the lens onto the retina, excluding peripheral light. This allows for a clearer image to be formed and substitutes for eyeglasses [6].

If the patient cannot read the chart, he or she should be moved closer to the chart until he or she is 5 feet away. If he or she is still unable to read the chart, the examiner should test vision by having the patient count fingers at 2 feet. If the patient cannot count fingers accurately, the next step is to test whether he or she can detect hand motion. If the patient is unable to detect hand motion, the next step is to determine if he or she has light perception. The eye not being tested must be completely covered to test for light perception. If the patient can perceive light, the examiner can check to see if the patient can determine which direction the light is coming from (light perception with projection) or not (light perception without projection) [12]. If the patient is unable to detect light in an eye, that eye is considered totally blind (no light perception) [6].

Near vision may be tested at the bedside with a near-vision card, held 14 inches from the patient’s face [12,15]. Poor performance on near-vision testing can be caused by acute visual impairment but may also be caused by presbyopia [16], which is age-related chronic impairment of near vision [12].

External examination

To begin the physical examination of the eye, one should start with the external structures. The eyelids and surrounding areas should be examined for swelling, erythema, warmth, skin growths, and tenderness. Trauma or infection (periorbital cellulites or zoster) may become apparent on evaluation of the peri-orbital region.

Proptosis can easily be identified with gross inspection of the orbits. Unilateral proptosis may be caused by orbital cellulitis, orbital hemorrhage, cavernous sinus thrombosis, or tumors. Bilateral exophthalmos is most commonly caused by hyperthyroidism [12]. Proptosis can be quantified by the ophthalmologist using an exophthalmometer [14]. Enophthalmos may be caused by an orbital blow-out fracture. The bony structures should be palpated to ascertain tenderness [6].

Lid evaluation may find ptosis, which may be congenital or acquired. Newly acquired ptosis may represent Horner’s syndrome, third nerve palsy, botulism, or myasthenia gravis [10]. Inability to close the lids may result from weakness of the orbicularis oculi muscle, as in seventh cranial nerve palsy. It is important to flip the upper lid if you are searching for a foreign body. Careful evaluation of the lids may reveal infections of the lid (lid

cellulitis) or dacrocystitis, which is an infection of the tear collection system (puncta, canaliculi, nasolacrimal duct). Other lid findings include hordeolum and chalazion. A hordeolum is an acute inflammation of a meibomian gland, and a chalazion is a chronic obstruction and inflammation of a meibomian gland [10,11].

Extraocular movements

When testing extraocular movement, special note should be made of the range, symmetry, smoothness, and speed of the eye movements. Nystagmus can also be observed. To test these movements, the patient is asked to fixate on a target with both eyes. The examiner then moves the target in four directions [6]. Disorders of ocular motility may be caused by cranial nerve dysfunction, extraocular muscle entrapment, or increased pressure within the orbit (orbital cellulitis or hemorrhage).

Examination of the conjunctiva

It is important to evaluate the conjunctiva lining the lids (palpebral conjunctiva) and the conjunctiva on the surface of the eye (bulbar conjunctiva). Conjunctival injection (prominence of vessels) may be di use or perilimbal (radiating outward from the limbus, which is where the cornea meets the sclera). Di use conjunctival injection usually results from inflammation or infection within the conjunctiva itself (conjunctivitis), whereas perilimbal injection may be the result of inflammation or infection within the uvea or anterior chamber. Chemosis refers to edema of the conjunctiva [11]. Conjunctivitis will be discussed in detail in the article by Mahmood and Narang found elsewhere in this issue.

Examination of the conjunctiva may also reveal a subconjunctival hemorrhage, which may be spontaneous or traumatic. Although these usually are benign and self-limited, severe subconjunctival hemorrhages (360 of bulbar conjunctiva) may be secondary to ruptured globe or coagulopathy [10].

Examination of the sclera and episclera

Inflammation of the episclera and sclera may be di cult to distinguish from conjunctivitis. Episcleritis is usually painless and causes injection of a sector of episcleral vessels. These vessels will blanche with application of phenylephrine. Vision usually is preserved. It is self-limited and may be associated with collagen vascular diseases [11].

Scleritis causes injection of scleral vessels with a characteristic violaceous color. It is painful and often causes decreased vision. The injection will not clear with topical phenylephrine. It is associated with collagen vascular diseases and certain infections (zoster, tuberculosis, syphilis). Treatment depends on severity but often includes systemic steroids [11]. See the article by Mahmood and Narang in this issue for detailed discussion of scleritis and episcleritis.